The effect of small changes in rate of force development on muscle fascicle velocity and motor unit discharge behaviour.

When rate of force development is increased, neural drive increases. There is presently no accepted explanation for this effect. We propose and experimentally test the theory that a small increase in rate of force development increases medial gastrocnemius fascicle shortening velocity, reducing the muscle's force-generating capacity, leading to active motor units being recruited at lower forces and with increased discharge frequencies. Participants produced plantar flexion torques at three different rates of force development (slow: 2% MVC/s, medium: 10% MVC/s, fast: 20% MVC/s). Ultrasound imaging showed that increased rate of force development was related to higher fascicle shortening velocity (0.4 ± 0.2 mm/s, 2.0 ± 0.9 mm/s, 4.1 ± 1.9 mm/s in slow, medium, fast, respectively). In separate experiments, medial gastrocnemius motor unit recruitment thresholds and discharge frequencies were measured using fine-wire electromyography (EMG), together with surface EMG. Recruitment thresholds were lower in the fast (12.8 ± 9.2% MVC) and medium (14.5 ± 9.9% MVC) conditions compared to the slow (18.2 ± 8.9% MVC) condition. The initial discharge frequency was lower in the slow (5.8 ± 3.1 Hz) than the fast (6.7 ± 1.4 Hz), but not than the medium (6.4 ± 2.4 Hz) condition. The surface EMG was greater in the fast (mean RMS: 0.029 ± 0.017 mV) compared to the slow condition (0.019 ± 0.013 mV). We propose that the increase in muscle fascicle shortening velocity reduces the force-generating capacity of the muscle, therefore requiring greater neural drive to generate the same forces.

Fine-wire recordings of flexor hallucis brevis motor units up to maximal voluntary contraction reveal a flexible, nonrigid mechanism for force control.

Our current knowledge on the neurophysiological properties of intrinsic foot muscles is limited, especially at high forces. This study therefore aimed to investigate the discharge characteristics of single motor units in an intrinsic foot muscle, namely flexor hallucis brevis, during voluntary contractions up to 100% of maximal voluntary contraction. We measured the recruitment threshold and discharge rate of flexor hallucis brevis motor units using indwelling fine-wire electrodes. Ten participants followed a target ramp up to maximal voluntary contraction by applying a metatarso-phalangeal flexion torque. We observed motor unit recruitment thresholds across a wide range of isometric forces (ranging from 10 to 98% of maximal voluntary contraction) as well as across a wide range of discharge rates (ranging from 4.8 to 23.3 Hz for initial discharge rate and 9.5 to 34.2 Hz for peak discharge rate). We further observed patterns of high variability in recruitment threshold and discharge rate as well as crossover in discharge rate between motor units within the same participant. These findings suggest that the force output of a muscle is generated through a mechanism with substantial variability rather than relying on a rigid organization, which is in contrast to the proposed onion-skin theory. The demands placed on the plantar intrinsic foot muscles during high- and low-force tasks may explain these observed neurophysiological properties.NEW & NOTEWORTHY We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

The repeated bout effect can occur without mechanical and neuromuscular changes after a bout of eccentric exercise.

Changes in muscle fascicle mechanics have been postulated to underpin the repeated bout effect (RBE) observed following exercise-induced muscle damage (EIMD). However, in the medial gastrocnemius (MG), mixed evidence exists on whether fascicle stretch amplitude influences the level of EIMD, thus questioning whether changes in fascicle mechanics underpin the RBE. An alternative hypothesis is that neural adaptations contribute to the RBE in this muscle. The aim of this study was to investigate the neuromechanical adaptations during and after repeated bouts of a highly controlled muscle lengthening exercise that aimed to maximize EIMD in MG. In all, 20 subjects performed two bouts of 500 active lengthening contractions (70% of maximal activation) of the triceps surae, separated by 7 days. Ultrasound constructed fascicle length-torque (L-T) curves of MG, surface electromyography (EMG), maximum torque production, and muscle soreness were assessed before, 2 hours and 2 days after each exercise bout. The drop in maximum torque (4%) and the increase in muscle soreness (24%) following the repeated bout were significantly less than following the initial bout (8% and 59%, respectively), indicating a RBE. However, neither shift in the L-T curve nor changes in EMG parameters were present. Furthermore, muscle properties during the exercise were not related to the EIMD or RBE. Our results show that there are no global changes in gastrocnemius mechanical behavior or neural activation that could explain the observed RBE in this muscle. We suggest that adaptations in the non-contractile elements of the muscle are likely to explain the RBE in the triceps surae.

The effect of cadence on the muscle-tendon mechanics of the gastrocnemius muscle during walking.

Humans naturally select a cadence that minimizes metabolic cost at a constant walking velocity. The aim of this study was to examine the effects of cadence on the medial gastrocnemius (MG) muscle and tendon interaction, and examine how this might influence lower limb energetics. We hypothesized that cadences higher than preferred would increase MG fascicle shortening velocity because of the reduced stride time. Furthermore, we hypothesized that cadences lower than preferred would require greater MG fascicle shortening to achieve increased muscle work requirements. We measured lower limb kinematics and kinetics, surface electromyography of the triceps surae and MG fascicle length, via ultrasonography, during walking at a constant velocity at the participants' preferred cadence and offsets of ±10%, ±20%, and ±30%. There was a significant increase in MG fascicle shortening with decreased cadence. However, there was no increase in the MG fascicle shortening velocity at cadences higher than preferred. Cumulative MG muscle activation per minute was significantly increased at higher cadences. We conclude that low cadence walking requires more MG shortening work, while MG muscle and tendon function changes little for each stride at higher cadences, driving up cumulative activation costs due to the increase in steps per minute.

Effects of muscle activation on shear between human soleus and gastrocnemius muscles.

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles in vivo, we examined whether activation could alter the shear between the soleus (SOL) and lateral gastrocnemius (LG) muscles. We hypothesized that selective activation of LG would decrease the stretch-induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater (P < 0.05) and relative displacement of aponeuroses was smaller than during passive stretch (P < 0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion (P < 0.05). The activity-induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission.

Deconstructing the power resistance relationship for squats: A joint-level analysis.

Generating high leg power outputs is important for executing rapid movements. Squats are commonly used to increase leg strength and power. Therefore, it is useful to understand factors affecting power output in squatting. We aimed to deconstruct the mechanisms behind why power is maximized at certain resistances in squatting. Ten male rowers (age = 20 ± 2.2 years; height = 1.82 ± 0.03 m; mass = 86 ± 11 kg) performed maximal power squats with resistances ranging from body weight to 80% of their one repetition maximum (1RM). Three-dimensional kinematics was combined with ground reaction force (GRF) data in an inverse dynamics analysis to calculate leg joint moments and powers. System center of mass (COM) velocity and power were computed from GRF data. COM power was maximized across a range of resistances from 40% to 60% 1RM. This range was identified because a trade-off in hip and knee joint powers existed across this range, with maximal knee joint power occurring at 40% 1RM and maximal hip joint power at 60% 1RM. A non-linear system force-velocity relationship was observed that dictated large reductions in COM power below 20% 1RM and above 60% 1RM. These reductions were due to constraints on the control of the movement.

Recruitment order of the abdominal muscles varies with postural task.

Abdominal muscle recruitment strategies in response to a postural perturbation contradict the theory that the deeper abdominal muscles are always recruited in advance of the more superficial muscles. The purpose of this study was to determine whether such contrasting muscle recruitment patterns are due to the postural task or the predictability of a postural task. Participants performed an arm raise task as well as an unpredictable and a predictable balance perturbation task (i.e. support-surface translation) while intramuscular electromyographic (EMG) recordings were obtained from the deep [transversus abdominis (TrA)] and superficial [obliquus externus (OE)] abdominal muscles. The abdominal muscle recruitment order was dependent on the postural task but not on the predictability of a postural perturbation. Whereas arm raises elicited similar EMG onset latencies in TrA and OE, the OE onset latency was 48 ms earlier than the TrA following an unpredictable translation (P = 0.003). The early OE activation persisted when the translation was made predictable to the participant (P = 0.024). These results provide evidence that the abdominal muscle recruitment order varies with the trunk stability requirements specific to each task. Rehabilitation strategies focusing on an early TrA activation to improve postural stability may not be appropriate for all everyday tasks.

Stepping onto the unknown: reflexes of the foot and ankle while stepping with perturbed perceptions of terrain.

Unanticipated variations in terrain can destabilize the body. The foot is the primary interface with the ground and we know that cutaneous reflexes provide important sensory feedback. However, little is known about the contribution of stretch reflexes from the muscles within the foot to upright stability. We used intramuscular electromyography measurements of the foot muscles flexor digitorum brevis (FDB) and abductor hallucis (AH) to show for the first time how their short-latency stretch reflex response (SLR) may play an important role in responding to stepping perturbations. The SLR of FDB and AH was highest for downwards steps and lowest for upwards steps, with the response amplitude for level and compliant steps in between. When the type of terrain was unknown or unexpected to the participant, the SLR of AH and the ankle muscle soleus tended to decrease. We found significant relationships between the contact kinematics and forces of the leg and the SLR, but a person's expectation still had significant effects even after accounting for these relationships. Motor control models of short-latency body stabilization should not only include local muscle dynamics, but also predictions of terrain based on higher level information such as from vision or memory.

Increases in corticospinal responsiveness during a sustained submaximal plantar flexion.

Studying the responsiveness of specific central nervous system pathways to electrical or magnetic stimulation can provide important information regarding fatigue processes in the central nervous system. We investigated the changes in corticospinal responsiveness during a sustained submaximal contraction of the triceps surae. Comparisons were made between the size of motor-evoked potentials (MEPs) elicited by motor cortical stimulation and cervicomedullary motor-evoked potentials (CMEPs) elicited by magnetic stimulation of the descending tracts to determine the site of any change in corticospinal responsiveness. Participants maintained an isometric contraction of triceps surae at 30% of maximal voluntary contraction (MVC) for as long as possible on two occasions. Stimulation was applied to the motor cortex or the cervicomedullary junction at 1-min intervals during contraction until task failure. Peripheral nerve stimulation was also applied to evoke maximal M waves (M(max)) and a superimposed twitch. Additionally, MEPs and CMEPs were evoked during brief contractions at 80%, 90%, and 100% of MVC as a nonfatigue control. During the sustained contractions, MEP amplitude increased significantly in soleus (113%) and medial gastrocnemius (108%) muscles and, at task failure, matched MEP amplitude in the prefatigue MVC ( approximately 20-25% M(max)). In contrast, CMEP amplitude increased significantly in medial gastrocnemius (51%), but not in soleus (63%) muscle and, at task failure, was significantly smaller than during prefatigue MVC (5-6% M(max) vs. 11-13% M(max)). The data indicate that cortical processes contribute substantially to the increase in corticospinal responsiveness during sustained submaximal contraction of triceps surae.

Corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths.

This study investigated corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths. Similar investigations have been made on upper limb muscles, where evoked responses have been shown to increase up to approximately 50% of maximal force and then decline. We elicited motor-evoked potentials (MEPs) and cervicomedullary motor-evoked potentials (CMEPs) in the soleus (Sol) and medial gastrocnemius (MG) muscles using magnetic stimulation over the motor cortex and cervicomedullary junction during voluntary plantar flexions with the torque ranging from 0 to 100% of a maximal voluntary contraction. Differences between the MEP and CMEP were also investigated to assess whether any changes were occurring at the cortical or spinal levels. In both Sol and MG, MEP and CMEP amplitudes [normalized to maximal M wave (Mmax)] showed an increase, followed by a plateau, over the greater part of the contraction range with responses increasing from approximately 0.2 to approximately 6% of Mmax for Sol and from approximately 0.3 to approximately 10% of Mmax for MG. Because both MEPs and CMEPs changed in a similar manner, the observed increase and lack of decrease at high force levels are likely related to underlying changes occurring at the spinal level. The evoked responses in the Sol and MG increase over a greater range of contraction strengths than for upper limb muscles, probably due to differences in the pattern of motor unit recruitment and rate coding for these muscles and the strength of the corticospinal input.

Evidence for reduced efficacy of the Ia-pathway during shortening plantar flexions with increasing effort.

To determine whether the soleus (SOL) H-reflex is modulated during shortening contractions in a manner that has been observed for isometric contractions, SOL H-reflexes and M-waves were elicited via percutaneous electrical stimulation to the tibial nerve at an intensity that evoked an H-reflex at 50% of its maximum in 11 healthy subjects. Paired electrical stimuli were delivered as the ankle angle passed through 90 degrees at an interval of 400 ms while the subject performed shortening contractions at levels of plantar flexion torque ranging between 2 and 30% of that during a maximal voluntary contraction (MVC). H-reflexes were also recorded during the performance of isomeric contractions of plantar flexors at similar levels of plantar flexion torque and at the same joint angle (muscle length) in an additional five healthy subjects. Correlations were examined between the peak-to-peak amplitude of the first H-reflexes, M-waves and plantar flexion torques in both protocols. It was revealed that no significant correlation was found between the SOL H-reflex and increasing plantar flexion torque during shortening contractions (rho = -0.07, P = 0.15), while a strong positive correlation was observed for the isometric conditions (rho = 0.99, P < 0.01). No significant change was observed in the SOL M-wave for either contraction type. Furthermore, the H-reflexes elicited via paired stimuli with the same background activity in voluntary shortening contractions showed almost identical amplitudes, suggesting that the level of homosynaptic post-activation depression did not change in response to the varying levels of activation in voluntary shortening contractions. Therefore, the lack of increase in the H-reflex during shortening contractions at increasing intensities is possibly due to a centrally regulated increase in presynaptic inhibition. Such a downward modulation of the reflex suggests that Ia-excitatory input onto the SOL motoneurone pool needs to be reduced during the performance of shortening contractions.

Conditioning Ia-afferent stimulation reduces the soleus Hoffman reflex in humans when muscle spindles are assumed to be inactive.

Despite higher neural activation during active as compared to passive muscle shortening, Hoffman reflexes (H-reflexes) are similar. This may be explained by homosynaptic post-activation depression (HPAD) of Ia-afferents being present during active shortening. Accordingly, it was investigated whether conditioning electrical stimulation of the tibial nerve reduced the H-reflex less during active than passive shortening. The effects of two conditioning modes (0.2 and 1 Hz) were compared to a control mode without conditioning. H-reflexes and M-waves were elicited as the ankle passed 90 degrees with the soleus muscle undergoing passive or active (20% MVC) lengthening or shortening. Conditioning had no effect during active shortening. In contrast, during passive shortening, the H:M of the 1 Hz mode was significantly less than that of the 0.2 Hz and control modes. In lengthening, H:M was unaffected by conditioning. These findings support that HPAD reduces the synaptic efficacy of Ia-afferents during active shortening, active and passive lengthening, but not passive shortening.

In vivo measurement of the effect of intra-abdominal pressure on the human spine.

In humans, intra-abdominal pressure (IAP) is elevated during many everyday activities. This experiment aimed to investigate the extent to which increased IAP--without concurrent activity of the abdominal or back extensor muscles--produces an extensor torque. With subjects positioned in side lying on a swivel table with its axis at L3, moments about this vertebral level were measured when IAP was transiently increased by electrical stimulation of the diaphragm via the phrenic nerve. There was no electromyographic activity in abdominal and back extensor muscles. When IAP was increased artificially to approximately 15% of the maximum IAP amplitude that could be generated voluntarily with the trunk positioned in flexion, a trunk extensor moment (approximately 6 Nm) was recorded. The size of the effect was proportional to the increase in pressure. The extensor moment was consistent with that predicted from a model based on measurements of abdominal cross-sectional area and IAP moment arm. When IAP was momentarily increased while the trunk was flexed passively at a constant velocity, the external torque required to maintain the velocity was increased. These results provide the first in vivo data of the amplitude of extensor moment that is produced by increased IAP. Although the net effect of this extensor torque in functional tasks would be dependent on the muscles used to increase the IAP and their associated flexion torque, the data do provide evidence that IAP contributes, at least in part, to spinal stability.

Interaction between voluntary and postural motor commands during perturbed lifting.

STUDY DESIGN: An experimental study was conducted to evaluate the effect of an unexpected postural perturbation during a lifting task. OBJECTIVES: To investigate electromyographic responses in the erector spinae to a postural perturbation, simulating slipping, during an ongoing voluntary lifting movement. It was hypothesized that specific combinations of voluntary movement and postural perturbation present a situation in which injury caused by a rapid switch between conflicting motor commands can occur. SUMMARY OF BACKGROUND DATA: Studies of postural perturbations have mainly focused on behavior during static tasks such as quiet, upright standing. To date, there are no published studies of the effect of a perturbation during an ongoing voluntary lifting movement. METHODS: Subjects standing on a movable platform were exposed to random perturbations while lifting a 20-kg load. Muscle activity was recorded from flexor and extensor muscles of the trunk and hip. Trunk flexion angle in the sagittal plane was recorded with a video system. RESULTS: Perturbations forward were followed by an increased activity in erector spinae superimposed on the background activation present during the lift, indicating that both the voluntary and postural motor programs caused an activation of erector spinae. During backward perturbation, however, there was a sudden cessation of erector spinae activity followed by an extended period of rapid electromyographic amplitude fluctuations while the trunk was flexing, indicating an eccentric contraction of the erector spinae. CONCLUSIONS: This erratic behavior with large electromyographic amplitude fluctuations in the erector spinae after a backward slip during lifting may indicate a rapid switch between voluntary and postural motor programs that require conflicting functions of the back muscles. This may cause rapid force changes in load-carrying tissue, particularly in those surrounding the spine, thus increasing the risk of slip-and-fall-related back injuries.

Three dimensional preparatory trunk motion precedes asymmetrical upper limb movement.

Three-dimensional trunk motion, trunk muscle electromyography and intra-abdominal pressure were evaluated to investigate the preparatory control of the trunk associated with voluntary unilateral upper limb movement. The directions of angular motion produced by moments reactive to limb movement in each direction were predicted using a three-dimensional model of the body. Preparatory motion of the trunk occurred in three dimensions in the directions opposite to the reactive moments. Electromyographic recordings from the superficial trunk muscles were consistent with preparatory trunk motion. However, activation of transversus abdominis was inconsistent with control of direction-specific moments acting on the trunk. The results provide evidence that anticipatory postural adjustments result in movements and not simple rigidification of the trunk.

Upper body movement during walking in children with lumbo-sacral myelomeningocele.

Eight children with lumbo-sacral myelomeningocele (MMC) underwent three-dimensional movement analysis to determine whether or not differing levels of lower extremity strength affected the extent of shoulder, trunk and pelvis movement during independent walking when wearing orthoses. Fourteen control children were also investigated. The patterns of upper body movements in all MMC children were well defined and consistent, showing small standard deviations from the mean. In the frontal and transverse planes, segment displacements of the MMC children assigned into Group II (hip extensor and abductor muscle strength grade 0-2) were almost twice that of the MMC children in Group I (hip extensor and abductor muscle strength grade 3-4). All segment displacements in the frontal, transverse and sagittal planes for Group I and Group II children were significantly greater than those for the controls. In the frontal plane these differences were approximately 4-10 times greater, with the Group II children having the largest peak-to-peak displacements. These results indicate that the motion amplitudes of the upper body segments are related to the degree of muscle weakness of the lower limbs. No significant differences were found when comparing segment motions during walking with either the Ferrari type knee-ankle-foot or ankle-foot orthoses.

Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths.

We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and 27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths (P<0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle.

Muscle activation and torque development during maximal unilateral and bilateral isokinetic knee extensions.

BACKGROUND: The aim of this study was to investigate whether or not a bilateral strength deficit occurs during bilateral (BL) velocity controlled dynamic knee extensions and if the neural control of the knee extensors and flexors is altered during homologous muscle BL efforts. METHODS: Twenty-eight healthy and habitually active subjects, 13 female and 15 male, performed maximal unilateral (UL) and BL isokinetic leg extensions at a velocity of 60 degrees.s-1 through a 90 degrees range of motion of the knee joint (90 to 180 degrees). Knee extension torque and electromyographic activity (EMG) of the quadriceps and hamstrings muscles were recorded. RESULTS: The mean knee extensor torque produced in the BL condition (168 +/- 52 Nm) was 17% less than the sum of the two UL conditions (Sigma=202 +/- 56 Nm). During BL conditions, quadriceps EMG activity was less in both legs (left, 8.2 +/- 7.4% less and right, 13.9 +/- 9.1% less, respectively). There were no significant differences between BL and UL efforts for either left or right hamstrings activity. Eighteen subjects, who when asked to perform a maximal knee extension simultaneously activated their contralateral hamstrings, had significantly higher bilateral deficits (21%) compared to those who exhibited little or no contralateral hamstrings EMG activity (14%). CONCLUSIONS: The main findings of the study were that a bilateral strength deficit occurred when simultaneously maximally activating the homologous knee extensor muscles. This deficit was in all likelihood due to a less than maximal efferent drive to the quadriceps muscles. Hamstrings EMG activity was not greater during the BL knee extensions, which supports the notion that antagonistic muscle activity was not primarily responsible for the observed bilateral deficit.

Anticipatory postural activity of the deep trunk muscles differs between anatomical regions based on their mechanical advantage.

The functional differentiation between regions of psoas major (PM) and quadratus lumborum (QL) may underlie a mechanical basis for recruitment of motor units across the muscle. These mechanically unique fascicle regions of these complex multifascicular muscles, PM and QL, are likely to be controlled independently by the central nervous system (CNS). Fine-wire electrodes recorded the electromyographic activity of the PM fascicles arising from the transverse process (PM-t) and vertebral body (PM-v) and the anterior (QL-a) and posterior (QL-p) layers of QL on the right side during a postural perturbation associated with rapid arm movements. The findings of this study indicate that the CNS coordinates the activity of specific regions of PM and QL independently as a component of the anticipatory postural adjustments that precedes the predictable challenge to the spine associated with limb movements. The spatial and temporal features of discrete activity of different regions within PM and QL matched their differing mechanical advantage predicted from their anatomy. These findings suggest that the CNS differentially activates individual regions within complex spine muscles to control the three-dimensional forces applied to the spine. The data also point to a sophisticated control of muscle activation that appears based on mechanical advantage.

Muscle fascicle strains in human gastrocnemius during backward downhill walking.

Extensive muscle damage can be induced in isolated muscle preparations by performing a small number of stretches during muscle activation. While typically these fiber strains are large and occur over long lengths, the extent of exercise-induced muscle damage (EIMD) observed in humans is normally less even when multiple high-force lengthening actions are performed. This apparent discrepancy may be due to differences in muscle fiber and tendon dynamics in vivo; however, muscle and tendon strains have not been quantified during muscle-damaging exercise in humans. Ultrasound and an infrared motion analysis system were used to measure medial gastrocnemius fascicle length and lower limb kinematics while humans walked backward, downhill for 1 h (inducing muscle damage), and while they walked briefly forward on the flat (inducing no damage). Supramaximal tibial nerve stimulation, ultrasound, and an isokinetic dynamometer were used to quantify the fascicle length-torque relationship pre- and 2 h postexercise. Torque decreased ~23%, and optimal fascicle length shifted rightward ~10%, indicating that EIMD occurred during the damage protocol even though medial gastrocnemius fascicle stretch amplitude was relatively small (~18% of optimal fascicle length) and occurred predominantly within the ascending limb and plateau region of the length-torque curve. Furthermore, tendon contribution to overall muscle-tendon unit stretch was ~91%. The data suggest the compliant tendon plays a role in attenuating muscle fascicle strain during backward walking in humans, thus minimizing the extent of EIMD. As such, in situ or in vitro mechanisms of muscle damage may not be applicable to EIMD of the human gastrocnemius muscle.